1. Field of the Invention
The present invention relates to sensors, and more particularly, to image sensors.
2. Description of the Prior Art
Digital cameras are commonly used today. Typically, a digital camera contains image sensors which convert light into electrical charges. These image sensors can be divided into two broad categories according to the applied manufacturing process: CCD (Charge-Coupled Device) sensors; and CMOS (Complementary Metal Oxide Semiconductor) image sensors, where the CMOS image sensors (CIS) are based on CMOS technologies. Since the CMOS image sensors are compatible with typical CMOS fabrication processes, an integration of additional signal processing logic on the same substrate on which the sensor array is disposed is permitted.
In addition, the trend is for the pixel sizes of the CMOS image sensors to become smaller and smaller; however, these reduced sizes of pixels and pixel arrays affect the performance of the CIS system.
The CIS system traditionally applies Front Side Illumination (FSI) technology to form the pixels of the pixel array. In FSI CMOS image sensors, light travels to a photo-sensing area through the front side of the pixel. This means that the incident light has to first pass through dielectric layers, and metal layers before it strikes the photo-sensing area, causing low quantum efficiency (QE), serious cross talk between pixels, and dark current.
Another type of CMOS image sensor is BSI (Back Side illumination) CMOS image sensors. Back Side illumination technology was used in pixels of CCD image sensors. Instead of illuminating a CMOS image sensor from the top (front) side of the silicon die, a BSI CMOS image sensor applies the color filters and the microlens to the back side of the pixels so that the incident light is collected from the back side of the image sensor. Compared to the FSI CMOS image sensors, the BSI CMOS image sensors have less light loss, reduced crosstalk, and better quantum efficiency.
Both the FSI CMOS image sensors and BSI CMOS image sensors, however, need to improve their performance and reduce their disadvantages such as cross talk, and light loss. Therefore, there is a demand for the provision of CMOS image sensors with better performance.